As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, was proposed and is practically employed. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at a considerably smaller dose, as compared with the conventional radiography using a combination of a radiographic film and radiographic intensifying screen. Further, the radiation image recording and reproducing method using a stimulable phosphor is of great value especially when the method is employed for medical diagnosis.
The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a stimulable phosphor layer provided on one surface of the support. If the phosphor layer is self-supporting, however, the support may be omitted. Further, a transparent layer of a polymer material is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer generally comprises a binder and a stimulable phosphor (in the form of particles) dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when it is exposed to radiation such as X-rays and then excited with an electromagnetic wave (i.e., stimulating rays). Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the stimulable phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced on the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
As described hereinbefore, the surface on the stimulable phosphor layer (opposite the surface facing the support) is provided with a protective layer to protect the phosphor layer from chemical deterioration or physical damage. The protective layer can be provided, for instance, by coating a solution of a transparent organic polymer such as a cellulose derivative or polymethyl methacrylate on the phosphor layer, by fixing a beforehand prepared polymer film such as a polyethylene terephthalate film on the phosphor layer with an adhesive, or by vacuum depositing an inorganic material on the phosphor layer.
The beforehand prepared polymer film such as polyethylene terephthalate film has a high strength. However, it needs complicated procedures for its preparation. Moreover, if the adhesive layer between the polymer film and the phosphor layer gives two interfaces, that is, that between the adhesive layer and the polymer film, and that between the adhesive layer and the phosphor layer. The increased interfaces cause increase of scattering of light passing through these layers, and the increased scattering causes lowering of quality of an image obtained in the radiation image recording and reproducing method.
In contrast, the coated protective layer can be readily prepared by coating a solution of polymer material on the phosphor layer, and the coated protective layer is firmly fixed on the phosphor layer. Particularly, a protective layer prepared simultaneously with a phosphor layer by a simultaneous coating method is fixed on the phosphor layer with sufficient bonding strength, and moreover thus prepared radiation image storage panel shows improved sensitivity and image quality (U.S. Pat. No. 4,728,583). It has been found by the present inventors that the protective layer directly coated on the phosphor layer sometimes produces cracks therein in the steps of the radiation image recording and reproducing method, as described below.
In the radiation image recording and reproducing method, the radiation image storage panel is repeatedly employed in the steps of radiation of X-rays (recording of radiation image), irradiation of stimulating rays (reading out of the recorded radiation image), and exposure to erasing light (erasure of residual radiation image). Between these steps, the storage panel is transferred by conveyors such as belts and/or rollers within the apparatus for the radiation image recording and reproducing method. In these steps, the coated protective layer of the storage panel sometimes produces therein cracks, probably, due to its rigid body. Particularly, the coated protective layer of a fluororesin (i.e., fluorocarbonresin) showing high anti-staining properties which is described in copending U.S. Ser. No. 08/333,325 is so brittle as to produce cracks therein. The radiation image storage panel having a cracked protective layer cannot give a reproduced radiation image of high quality because X-rays or stimulating rays impinged on the cracked protective layer is scattered on the cracked.